Openings for doors, windows or cupboards etc, must be bridged over by some means to support the masonry above. The support to the load above the opening is usually given by providing an arch or a lintel. Lintel acts like a beam and transfers the vertically to the supporting walls. The end of lintel are built into the masonry so as to convey the weight carried by them to the masonry in jambs. The amount of bearing which the supports provide for the ends of lintel is an important factor. As a general rule, the bearing of a lintel at its ends should be 10 cm. (depth of one brick) or 38 cm. of span, whichever is greater.
Classification of Lintels Used in Building Construction
Various types of lintels are used in building construction are as follows:
- Wooden Lintels
In hilly areas, where timber suitable for the construction of lintel is easily available, wooden lintels are commonly used. In plains, the use of wooden lintel is very rare on account of high cost of the material. Timber if not properly ventilated, is liable to decay. Wood being combustible, wooden lintels can not be recommended for fire proof construction. The minimum depth of the lintel should be 7.6 cm. Sometimes a rough arch is constructed above a lintel to relieve the latter of the weight above. When used for large spans, the lintel should be properly designed as a timber beam simply supported at its ends.
- Stone Lintels
The use of stone lintels is very common is stone districts. Its use is usually confined to stone masonry structures or buildings faced with stones In general, stone lintel on account of its high cost and its inability to withstand excessive transverse stresses, is rarely used. The minimum thickness of stone lintel in kept 8 cm., and as a thumb rule, the thickness is taken as 4 cm., per 30 cm., length of span.
- Brick Lintels
They are constructed with hard, well brunt, first class brick laid on end or on edge. Depending upon the span, their depth varies from 10 cm. (depth of one brick) to 20 cm., length (length of one brick). They are considered suitable for small openings (generally not more than 90 cm., in span) with light loading conditions. Bricks having frogs are more suitable for construction of lintels. The frogs, when filled with mortar, form joggles which increase the shear resistance of the end joints and ultimately result in increased strength of lintel.
- Reinforced Brick Lintels
When brick lintels are required to be provided over large spans, they are reinforced with steel reinforcements. The depths of the lintel is limited to 10 cm. (depth of a brick) or a multiple of one brick thickness. Bricks are so arranged that 19 cm., to 38 cm., wide space is left length wise for the insertion of reinforcements. The gap is then filled with rich cement mortar or cement concrete.
- Reinforced Concrete Lintels
On account of their durability, strength and fire resisting properties, reinforced concrete lintels are very common in use. They are economical and easy in construction. They can be used for bigger spans and heavy loading conditions.
R.C.C. lintels are constructed with 1:2:4 cement concrete reinforced with steel bars. They may be precast or cast-in-situ. For smaller spans say up to 1.2 m, precast lintels are preferred.
Precast lintels are manufactured and cured well in advance and as such prove economical since the same mould can be used a number of times. The use of precast R.C.C. lintels expedites the construction. While laying a precast lintel special care should be taken to see that the marked face of the lintel (soon after pouring concrete in the mould, the top of the concrete surface is marked with some paint or far so as to distinguish it from bottom face) is kept at top. If the lintel is placed in reverse position i.e., the marked face pointing downward, disastrous result may take place. For larger spans, lintels are cast-in-situ, as precast lintels will have to provided with additional reinforcements to guard against the lifting stress. For lintels over large spans subjected to heavy loadings, the depth of the lintel and the area of the reinforcement should be accurately computed.
- Steel Lintels
When a large opening carry heavy wall load is to be spanned by a lintel, an reinforced concrete (R.C. LINTEL) will work out to be very deep. If due to architectural reasons, the depth of the lintel is to be restricted, it is common to use steel lintel. Steel lintel consists of section of rolled steel joists, used singly or in a combination of two or three. Built up lintel consists of three rolled steel joists (R.S.J’s) spaced together and connected to each other by bolts passing through them at intervals. This system is embedded in concrete and is cured like reinforced concrete (R.C.) lintel.